Method and apparatus for shrinking biaxially stretched thermoplastic films

Abstract

A thermoplastic film web is subjected to a defined shrinking in a treatment apparatus with two mirror-symmetrically arranged tentering chains. For this purpose, each chain is equipped with tentering clamps that have a variable on-center spacing or pitch in the travel direction of the tentering chains, and which additionally are position adjustable in a direction other than the travel direction. The entire clamp body or clamp elements may be position variable in a direction other than the travel direction. In operation, the pitch is first reduced in the inlet and/or heating zone of the treatment apparatus. Simultaneously, the clamping position of the individual clamps or clamping element is changed, e.g. by tilting or lifting. The clamp position change takes up extra film web length resulting from the pitch change, whereby the film web assumes throughout its width a corrugated configuration. Then, the film web is cross-stretched. Then the position of the clamps or clamp elements is changed in the setting zone of the apparatus so that the film web is plane again, whereby the shrinking takes up the corrugations. Then, the clamps release the film web and thereafter the original longer pitch is restored.

What I claim is: 
     
       1. A method for shrinking a biaxially stretched thermoplastic film in a treatment apparatus having two tentering chains arranged in a common plane, said tentering chains defining an inlet zone, a heating zone, a stretching zone, a setting zone, and an outlet zone in that order as viewed in the travel direction of said thermoplastic film, said tentering chains carrying tentering clamps for gripping an edge of said thermoplastic film as it enters said inlet zone and for carrying said thermoplastic film through said treatment apparatus, said method comprising the following steps: (a) gripping said thermoplastic film with said tentering clamps as said thermoplastic film enters said inlet zone of said apparatus,   (b) varying a pitch between neighboring tentering clamps, whereby a first on-center spacing (T) between neighboring tentering clamps is reduced in said travel direction to a second on-center spacing (t),   (c) simultaneously varying a starting position of said tentering clamps in a positional change direction other than said film travel direction whereby said tentering clamps assume a varied position so that an excess film length is provided that is defined in its size and position,   (d) cross-stretching said thermoplastic film in said stretching zone,   (e) changing said varied position of said tentering clamps back to said starting position in said setting zone,   (f) releasing said thermoplastic film from said tentering clamps, and   (g) returning said on-center spacing (t) back to said first on-center spacing (T).   
     
     
       2. The method of claim 1, wherein said simultaneous varying of said pitch and of said starting position of said tentering clamps is performed in said inlet zone. 
     
     
       3. The method of claim 1, wherein said simultaneous varying of said pitch and of said starting position of said tentering clamps is performed in said heating zone. 
     
     
       4. The method of claim 1, wherein said step (g) of returning said second on-center spacing (t) to said first on-center spacing (T) is performed after said releasing step and prior to said tentering clamps entering said outlet zone of said treatment apparatus. 
     
     
       5. The method of claim 1, wherein said step of returning said second on-center spacing (t) to said first on-center spacing (T) is performed after said releasing step in said outlet zone. 
     
     
       6. The method of claim 1, wherein said step of varying said starting position and said step of changing said varied position is performed by a tilting motion of said tentering clamps. 
     
     
       7. The method of claim 1, wherein said step of varying said starting position and said step of changing said varied position of said tentering clamps is performed by a shifting motion in said positional change direction and in an opposite positional change direction. 
     
     
       8. The method of claim 1, wherein said step of varying said starting position and said step of changing said varied position is performed by superimposing a tilting motion and a shifting motion on each other for positioning said tentering clamps. 
     
     
       9. The method of claim 1, wherein said varied position is maintained for a first duration that is a proportion of the time corresponding to one full revolution of said tentering chains. 
     
     
       10. The method of claim 9, wherein said second on-center spacing (t) is maintained for a second duration that is longer than said first duration. 
     
     
       11. An apparatus for shrinking a biaxially stretched thermoplastic film in a treatment apparatus, comprising two tentering chains arranged in a common plane, said tentering chains defining an inlet zone, a heating zone, a stretching zone, a setting zone, and an outlet zone in that order as viewed in a film travel direction of said thermoplastic film, said tentering chains carrying tentering clamps for gripping each edge of said thermoplastic film as it enters said inlet zone and for carrying said thermoplastic film through said treatment apparatus, said apparatus further comprising first means for varying a pitch between neighboring tentering clamps, whereby a first on-center spacing (T) between neighboring tentering clamps is reduced in said travel direction to a second on-center spacing (t), second means for simultaneously varying a starting position of said tentering clamps in a positional change direction other than said film travel direction whereby said tentering clamps assume a varied position so that an excess film length is provided that is defined in its size and position, third means for cross-stretching said thermoplastic film in said stretching zone, fourth means for changing said varied position of said tentering clamps back to said starting position in said setting zone, fifth means for releasing said thermoplastic film from said tentering clamps, and sixth means for returning said second on-center spacing (t) back to said first on-center spacing (T). 
     
     
       12. The apparatus of claim 11, wherein said tentering clamps comprise a clamp table and a clamp flap cooperating with each other for holding said thermoplastic film, said clamp table and said clamp flap defining an initial clamping plane corresponding to an inlet plane of said thermoplastic film, said apparatus further comprising means for varying an elevation of said initial clamping plane relative to said inlet plane. 
     
     
       13. The apparatus of claim 11, wherein said tendering clamps comprise a clamp table and a clamp flap cooperating with each other for holding said thermoplastic film, and means for tilting said clamp table and flap about an axis extending in parallel to said film travel direction.